The present invention is directed to providing controlled flow of a chemical reagent vapor. In particular, the present invention is directed to providing accurate, and reproducible flow of the chemical reagent vapor of a volatile liquid for use in the manufacture of semiconductor devices.
The accurate and reproducible control of reagent vapors to a reactor is critical to the manufacture of semiconductor devices, optoelectronic components, solid state lasers and numerous other high technology products. This is especially true for source materials (reagents) utilized in chemical vapor deposition (CVD) processes. In these processes, it is often necessary to introduce into the CVD reactor the vapor of volatile liquids, the vapor often being transported to the reactor in a carrier gas. For a number of less volatile source materials, maintaining the source container at elevated temperature to generate sufficient vapor pressure is not possible because of long term thermal decomposition of the source material. Safety is also a consideration in heating flammable liquids above their flash point in order to achieve sufficient vapor flow. These concerns are particularly true of many organometallic source materials.
An approach to the delivery of liquid source materials is the use of mass flow controllers employing Wheatstone Bridge type circuitry. These “thermal bridge” controllers are used extensively for delivering compressed gases, but lack the necessary sensitivity to measure and control a small vapor component diluted in a large flow of carrier gas. Additionally, the controller must be heated to avoid vapor condensation, but this decreases the temperature differential between the heated element in the sensor tube and the controller temperature, resulting in decreased flow sensitivity. A modification of this design for liquid use typically exhibits unacceptably long response time for step-function flow changes. This is due to the minute movement of liquid (unlike gases) through the sensor tube. Thermal decomposition of the source material in the heated sensor tube and consequent plugging and sensor drift also erode controller performance. The design, operation and performance limitations of the thermal bridge-type controllers are well known to those skilled in the art.
For the above reasons, it is desirable to avoid passing the source material through a flow sensor or controller. In this regard, and for more volatile-liquid source materials, it is common practice to utilize a bubbler-type source container and delivery system in which a carrier gas is bubbled through the liquid in the container; the carrier gas being saturated with the vapor of interest and resultant transport of the entrained vapor to the reactor. The relationship of the mass flow of vapor, the source container temperature, and the carrier gas flow rate are, to a first approximation, given by:       Mass    ⁢                  ⁢    flow    ⁢                  ⁢    of    ⁢                  ⁢    vapor    =                    M        ⁢                                  ⁢        W                    22        ⁣        ⁣                  ,          414                      ·                            P          0                ⁡                  (          T          )                            P        -                              P            0                    ⁢          T                      ·    f  where P is the total pressure in the headspace over the liquid in the bubbler; P0 is the vapor pressure of the neat liquid (or the total vapor pressure of several components in a liquid mixture), a function of the liquid temperature; f is the volume flow rate of carrier gas in standard cubic centimeters per minute (sccm), and MW is the gram-molecular weight of the vapor of interest. The theoretical and practical aspects of bubbler operation have been examined in, for example, A. Love, S. Middleman and A. Hochberg, “The Dyamics of Bubblers as Vapor Delivery Systems”, Jour. of Crystal Growth, 129 pages 119–133 (1993) and S. Middleman and A. K. Hochberg, Process Engineering Analysis in Semiconductor Device Fabrication, McGraw-Hill (1993).
A number of difficulties exist with the bubbler method that significantly diminish its reliability. For example, at high carrier gas flow rates, rapid removal of the vapor over the liquid leads to evaporative cooling of the liquid surface with a concomitant drop in vapor pressure. This causes nonlinearity in the vapor flow/carrier gas flow relationship. In general, this and other difficulties result from the sensitivity of the delivery system behavior to temperature and temperature gradients. Moreover, in order to avoid condensation of the vapor downstream of the source container, heat tracing of all delivery lines, valves, filters, etc. must be undertaken. This adds additional cost, complexity and maintenance to the bubbler-type delivery system. Yet another drawback of the bubbler-type system relates to the fractional distillation that occurs as the liquid of a multi-component mixture is evaporated. This leads to concern about the concentration of impurities even in ostensibly single component liquid sources.
The present invention essentially eliminates the weaknesses characteristic of both the thermal bridge type controller and the bubbler-type delivery system.